This invention relates to a shadow mask type color picture tube having an electron shield for shielding secondary electrons emitted from electron guns of the tube.
In a shadow mask type color picture tube, a shadow mask is fixedly positioned opposite to a phosphor screen by being supported by a support frame on an inner wall of a panel glass (an envelope) whose internal space is evacuated to a high degree of vacuum, and an electron shield for shielding secondary electrons is also fixed in position by the support frame along the outer peripheral edge of the shadow mask on the side of electron guns.
This electron shield is commonly produced by shaping an aluminum strip or foil having a thickness of 0.1 mm into the form by a press. The structure of such a color picture tube is disclosed in, for example, JP-U-52-42056 (Japanese Utility Model) filed in Japan by Hitachi, Ltd. on Sept. 19, 1975 and JP-A-58-198825 filed in Japan by Hitachi, Ltd. on May 14, 1982.
In some case, the function of such an electron shield is substituted by part of the support frame, that is, the electron shield is formed as an integral part of the support frame. In this case, a steel sheet having a thickness smaller than that usually used for making the conventional support frame is employed, and the support frame is formed by stamping or drawing the blank by means of a press. The support frame is disposed as close as possible to the inner wall surface of the panel glass, and the inner end of the support frame is extended as much as possible toward the interior of the panel glass so as to enhance the effect of secondary electron shielding.
Two forms of a color picture tube comprising such a light-weight support frame having an electron shield formed as its integral part as described above are disclosed in, for example, JP-B-59-1384 filed in Japan by Hitachi, Ltd. on Apr. 26, 1976 and JP-U-57-94160 filed in Japan by Hitachi, Ltd. on Dec. 1, 1980.
Generally, the surface of aluminum is oxidized by oxygen present in air and further hydrated by moisture present in air, so that the surface is turned into Al.sub.2 O.sub.3.3H.sub.2 O. Therefore, when such a strip or foil of aluminum is used to make the electron shield, the decomposition Al.sub.2 O.sub.3.3H.sub.2 O.fwdarw.Al.sub.2 O.sub.3 +3H.sub.2 O occurs when impinged by secondary electrons. Thus, 3H.sub.2 O, that is, moisture is liberated in the color picture tube thereby degrading the emission characteristic of the cathode of the electron guns.
On the other hand, in the case of the support frame made of a steel sheet of smaller thickness to include the electron shield as its integral part, the mechanical strength is inevitably lowered to reduce the howling level.
Also, when the electron shield is entirely eliminated, lead (Pb) in vapor form tends to be liberated from a frit glass by the action of combustion gases during the step of conveying the envelope through a frit baking oven for welding the panel glass to a funnel glass by the frit glass. This liberation of lead vapor is undesirable in that not only discoloration of the frit glass occurs, but also leakage tends to occur at the weld or implosion of the tube tends to occur during the step of evacuating the tube.